1. Field of the Invention
The present invention relates to a method of manufacturing a ferro-dielectric material capacitor and particularly to a method of manufacturing a ferro-dielectric material capacitor to be used in FeRAM (Ferroelectric Random Access Memory).
2. Description of Related Art
In these years, attention is paid to a FeRAM using a ferro-dielectric material film because of its excellent performance such as non-volatility and quick access time.
FIG. 10(A) schematically illustrates a cross-sectional structure of a ferro-dielectric material capacitor used in the FeRAM of the related art. In this figure, 101 designates a silicon substrate; 102, an insulation film used for element isolation or interlayer insulation; 103, a lower electrode of capacitor formed on the insulation film; 104, a ferro-dielectric material capacitor insulation film; 105, an upper electrode of capacitor.
In general, since oxygen easily becomes reduced in amount in the ferro-dielectric material film due to the process using the reduction atmosphere in the FeRAM manufacturing process, the annealing process in the oxidation atmosphere must be conducted adequately in the course of the process.
In this case, if the upper electrode and lower electrode are oxidized and an oxide film is formed thereon, a dielectric coefficient of the capacitor insulation film of the ferro-dielectric material capacitor as a whole is lowered and the ferro-dielectric characteristic is also deteriorated as represented by deterioration of the polarizing characteristic.
In view of solving such disadvantage, a film of noble metal such as Pt which is not easily oxidized and a film of Ir and Ru which does not lose conductivity when it is oxidized are used as the lower electrode and upper electrode in FeRAM.
The structure of the capacitor insulation film illustrated in FIG. 10(A) is formed in the following processes.
First, Pt as the lower electrode material of the capacitor, PZT as the ferro-dielectric material and Pt film as the upper electrode material are sequentially formed on the insulation film 102 such as a silicon oxide film formed on the silicon substrate 101.
Next, the upper electrode material is etched to form an upper electrode 105 by the dry etching method using the resist formed in the shape of a pattern of the upper electrode as the mask.
Next, after removing the resist by ashing it using the plasma of gas including oxygen, the resist film is formed in the shape of a pattern of the capacitor insulation film and this resist film is used as the mask to form the capacitor insulation film 104 by etching such ferro-dielectric material with the dry etching method.
Next, the resist film that has been used as the mask is removed by ashing it using the plasma of the gas including oxygen and subsequently, the resist film is formed in the shape of a pattern of the lower electrode. This resist film is then used as the mask to perform the etching of layer 103a (FIG. 10B) by the dry etching method to form the lower electrode 103. Thereafter, the resist film used as the mask is removed with the ashing process.
FIG. 10(B) is given to explain the problems in the method of forming the capacitor structure explained above. As illustrated in FIG. 10(B), when the ferro-dielectric material is etched with the dry-etching method using a mask 106 such as the resist, a reactive byproduct 107 is generated. Such byproduct will cause a leak of that capacitor and also gives adverse effect on the element, for example, it will cause an irregular pattern when it is peeled in the post-process and therefore such byproduct 107 must be removed after the etching process.
As a method of removing such reaction byproduct, a method of using hydrochloric acid and nitric acid and a method of using organic solvent have been proposed.
However, when such hydrochloric acid and nitric acid are used, the etching rate of the ferro-dielectric material as the capacitor insulation film is high and therefore it is difficult to control the amount of etching, resulting in the problem that the ferro-dielectric material has been etched too much. Particularly, while the element is micro-miniaturized, such reduction of the film has become a serious problem.
Moreover, in the method of using an organic solvent, a problem that the ferro-dielectric material is etched does not occur, but it has become apparent that a problem that such organic solvent penetrates into the area between the ferro-dielectric material film and the lower electrode of Pt, Ir, Ru or the like to provide peeling of the ferro-dielectric material film.
Moreover, it is considered, as the other approach to remove the reaction byproduct, to make it difficult to deposit the reaction byproduct or make it easier to remove the reaction byproduct by forming a tapered shape of the cross-section of the ferro-dielectric material film after the etching process. However, even when such cross-section is formed into the shape which is a little tapered, it is impossible to attain the effect to prevent deposition of the reaction byproduct and it has been difficult to obtain the desired tapered shape with good controllability while satisfying various properties such as selection ratio of the etching for the lower electrode, shape distribution within the wafer and productivity influenced by the etching rate.
The present invention discloses a method that has solved the problems explained above and can remove reaction byproduct after removing the ferro-dielectric material film with good controllability.
The problems explained above can be solved by etching the ferro-dielectric material film formed on a semiconductor substrate and thereafter such ferro-dielectric material film is wetted with an aqueous solution of phosphoric acid.
In this case, a resist may be used as the etching mask and the problem can be solved with the wet processing using the phosphoric acid after the resist ashing process.
Particularly, when the resist is used as the mask, the reaction byproduct, when it is produced in a large amount, can be removed successfully by conducting the wet process using the aqueous solution of phosphoric acid before and after the resist ashing process.
FIG. 11 is a schematic illustration of the capacitor structure after the process using hydrochloric acid. This schematic illustration is drawn based on the SEM photograph of the cross-section of the capacitor structure after the process using the hydrochloric acid. The PZT film is formed as the ferro-dielectric material on the Pt lower layer electrode and the PZT film is etched into the shape of the capacitor insulation film by the dry etching method using the resist as the mask. Thereafter, the resist is removed with the ashing process in the gas atmosphere including oxygen and fluorine and thereafter the process using the hydrochloric acid has been conducted.
As will be understood from the figure, the PZT film is reduced to a large extent by the process using the hydrochloric acid and the etching is conducted in such a degree to form up-diffusion with the upper electrode. It is also true when nitric acid is used. Here, the processing conditions such as concentration or the like have been searched to find out that there is no difference in very large etching rate of the PZT film and remarkable reduction of film.
FIGS. 12(A) and 12(B) schematically illustrate the capacitor structure before and after the process using phosphoric acid. This schematic illustration is drawn based on the SEM photograph of the cross-section. As in the case of the experiment of the hydrochloric acid process, the conditions of cross-section of the capacitor structure after the resist ashing removal and after subsequent phosphoric acid process, following the etching of the PZT film into the shape of the capacitor insulation film, are illustrated.
As will be understood from this figure, the reaction byproduct can be removed perfectly by using phosphoric acid. Moreover, as illustrated in FIG. 11, reduction of PZT film cannot be observed. Accordingly, the reaction byproduct due to the etching of the capacitor insulation film material can be removed without resulting in the influence on the capacitor shape particularly, capacitor insulation film) by introducing the phosphoric acid process.